Last night I saw The Monster Squad at a Halloween party, and in the part where the Army is called in for help, a tank plus several jeeploads of men arrive. Other movies show that happening in scenes where there is sudden big trouble and the military has to be called in.

Of course it's Hollywood. So what is done in real life situations where the military has to be called in?

Typically it is the National Guard that is called in, generally by the State Governor. This means reporting to armories, drawing equipment, etc. A good example of this was in LA during the Rodney King riots, IIRC, many of the guardsmen were unable to draw full ammo loads, as the ammo was expended in Desert Storm and had not been replaced.

To deploy the Army or Marines would require the President's authority, although once that came they could deploy quicker. The best recent example would be the deployment of the Armed Forces following 9/11.